Utilizing a dinuclear iron center tyrosyl radical cofactor, ribonucleoside diphosphate reductase (RNR) from E. coli catalyzes the conversion of nucleoside diphosphates to deoxynucleoside diphosphates. 2'-(E)-Fluromethylene-2'-deoxycytidine 5'-diphosphate (FMCDP) has recently been shown to be a near stoichiometric mechanism based inhibitor for this enzyme. Loss of the essential tyrosyl radical and formation of a new nucleotide based radical accompany inactivation. In order to gain insight into the structure of this radical, [6'-13C]-FMCDP has been synthesized, and the EPR spectrum of the radical resulting from its incubation with RDPR has been resolved. Simulation of the resulting spectrum provides unambiguous evidence that this new radical is allylic in nature, and is derived from H0133'-hydrogen atom abstraction mediated by RDPR.